Udpate build code

Main.hs

@@ -57,6 +57,7 @@ newtype VersionsSpec = VersionsSpec
 
 getVersionsSpec :: IO VersionsSpec
 getVersionsSpec = do
+    -- TODO: if doesn't exist: run niv init
     putStrLn $ "Reading versions file"
     decodeFileStrict pathNixVersionsJson >>= \case
       Just (Aeson.Object obj) ->

nix/default.nix

@@ -1,12 +1,3 @@
 with { fetch = import ./fetch.nix; };
 import fetch.nixpkgs
-  { overlays =
-      [ (self: super:
-          {
-            snack-exe = (import fetch.snack).snack-exe;
-            snack-lib = (import fetch.snack).snack-lib;
-          }
-        )
-      ] ;
-    config = { } ;
-  }
+  { overlays = import ./overlay.nix { sources = fetch; } ; config = {}; }

nix/overlay.nix

@@ -0,0 +1,13 @@
+{ sources ? import ./fetch.nix }:
+[
+  (self: super:
+    {
+      snack-exe = (import sources.snack).snack-exe;
+      snack-lib = (import sources.snack).snack-lib;
+    }
+  )
+
+  (self: super:
+    { niv = super.snack-lib.inferHPackBuild ../package.yaml ; }
+  )
+]